The old (joke) song title: How can I miss you if you won't go away?
I don't miss GT4 because I'm still playing it as much as or more than GT5.
The many failures and shortcomings of GT5 have been discussed to death elsewhere, as well as its few good points, but I'll mention one of my favorite gripes.
I find it relaxing to go tooling around in a Lotus Elan or something similar. The other day I did two races, in two separate series, on Nurburgring. Two weeks ago or so I'd driven a somewhat longer race with the Elan on the 'Ring, along with races on Opera Paris, Monaco and la Sarthe. Great fun.
Now with GT5, outside of Arcade mode and a one-make race (neither of which pay anything, by the way) I can't do that. My options are the GTWC, GT All Stars, Dream Car Championship and the Nurb enduro. The Elan isn't even remotely competitive in any of these. This isn't an isolated case, either, there are numerous other cars/lack of races.
On the other hand, I can finally race the BMW M3 GTR Race Car in a series it belongs in!
Still and all, when and if my PS2 and/or GT4 disk expires, I'll be looking for a replacement.